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Anchorage for plants Medium for Water & Air Circulation Reservoir for Water & Nutrients Space for beneficiary Micro Organisms Inter relationship between soil pores and its water holding capacity Plant water absorption rate Why study Soil water

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Soil Structure in relation to water movement

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Role of Structure in Irrigation Management Vital role in Soil Air & Water system In surface soil str., associated with soil tilth, permeability of Water Air & penetration of roots Soil porosity bulk density etc… Promotes all plant growth factors

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Porosity (  ) Typical values: 30 - 60%

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Soil Classification Alluvial soils F ormed by successive deposition of silt transported by rivers during floods, in the flood plains and along the coastal belts. Alluvial soils textures vary from clayey loam to sandy loam. The water holding capacity of these soils is fairly good and is good for irrigation.

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Black soils Weathering of rocks such as basalts, traps, granites and gneisses. Found in Maharashtra, MP, AP, Gujarat and TN Heavy textured with the clay content varying from 40 to 60 % High water holding capacity but poor in drainage. Red soils Formed by the weathering of igneous and metamorphic rock comprising gneisses and schist’s. Found in Tamil Nadu, Karnataka, Goa, Daman & Diu, south-eastern Maharashtra, Eastern Andhra Pradesh, Orissa and Jharkhand. The red soils have low water holding capacity and hence well drained.

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Laterites and Lateritic soils Laterite is a formation peculiar to India and some other tropical countries, with an intermittently moist climate. Found in Karnataka, Kerala, Madhya Pradesh, Eastern Ghats of Orissa, Maharashtra, West Bengal, Tamilnadu and Assam. These soils have low clay content and hence possess good drainage Desert soils Found in Western Rajasthan, Haryana, and Punjab, Poor soil development. Light textured sandy soils and react well to the application of irrigation water. •

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Problem soils Cannot be used for the cultivation of crops without adopting proper reclamation measures. Highly eroded soils, ravine lands, soils on steeply sloping lands etc. constitute one set of problem soils. Acid, saline and alkaline soils constitute another set of problem soil.

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(g) (g) (cm 3 ) (cm 3 ) Equivalent Depth

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Coarse Sand Silty Clay Loam Gravitational Water Water Holding Capacity Available Water Unavailable Water Dry Soil

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Soil Water Potential Description Measure of the energy status of the soil water Important because it reflects how hard plants must work to extract water Units of measure are normally bars or atmospheres Soil water potentials are negative pressures (tension or suction) Water flows from a higher (less negative) potential to a lower (more negative) potential

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Components  t = total soil water potential  g = gravitational potential (force of gravity pulling on the water)  m = matric potential (force placed on the water by the soil matrix – soil water “tension”)  o = osmotic potential (due to the difference in salt concentration across a semi-permeable membrane, such as a plant root) Matric potential,  m , normally has the greatest effect on release of water from soil to plants Soil Water Potential

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Soil Water Release Curve Curve of matric potential (tension) vs. water content Less water  more tension At a given tension, finer-textured soils retain more water (larger number of small pores)

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Height of capillary rise inversely related to tube diameter Matric Potential and Soil Texture The tension or suction created by small capillary tubes (small soil pores) is greater that that created by large tubes (large soil pores). At any given matric potential coarse soils hold less water than fine-textured soils.

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Classification of Soil Water Gravitational water – Excess water in soil pores – drains out due to gravitational force – Not available for plant growth Capillary water – Water left out in capillary pores after excess water has drained – Held by surface tension – cohesive force 1/3-15 atmp. – Available to plants Hygroscopic water – Water absorbed by a oven dry soil when exposed to a moist air – Held at high tension - tightly held by adhesion force – water of adhesion 10000-31 atmp., water not available – permanent wilting point

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Soil water constants Soil water proportions which dictate whether the water is available or not for plant growth. Saturation capacity: W ater content of the soil when all the pores of the soil are filled with water. (Maximum water holding capacity) So il moisture tension almost equal to zero. Field capacity: W ater retained by an initially saturated soil against the force of gravity. At field capacity, the macro-pores of the soil are drained off, but water is retained in the micropores. Soil Moisture tension at field capacity varies from 1/10 (for clayey soils) to 1/3 (for sandy soils) atmospheres.

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Field Capacity (FC or  fc ) Soil water content where gravity drainage becomes negligible Soil is not saturated but still a very wet condition Traditionally defined as the water content corresponding to a soil water potential of 2.54 (PF) Permanent Wilting Point (WP or  wp ) Soil water content beyond which plants cannot recover from water stress (dead) Still some water in the soil but not enough to be of use to plants Traditionally defined as the water content corresponding to -15 bars of SWP (pF 4.2)

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Permanent wilting point As the Plants extract water, the moisture content diminishes and the negative (gauge) pressure increases. At one point, the plant cannot extract any further water and thus wilts. Temporary wilting point: this denotes the soil water content at which the plant wilts at day time, but recovers during night or when water is added to the soil. Ultimate wilting point: The plant wilts and fails to regain life even after addition of water to soil.

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Available Water Definition Water held in the soil between field capacity and permanent wilting point “Available” for plant use Available Water Capacity (AWC) AWC =  fc -  wp Units: depth of available water per unit depth of soil, “unitless” (in/in, or mm/mm) Measured using field or laboratory methods

Water requirements of crops:

Points to remember:

Points to remember Cropped field acts as soil – water reservoir Residual soil moisture and shallow water table contributes to crop water need Water added in excess lost as – deep percolation - lead to nutrient loss, water logging and salinity Soils classified based on texture Water retention capacity differ with soils

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FC-upper limit of soil water storage Soil water content between FC and PWP- is total ASW for plant growth Crops differ in ability to withstand diff. depletion of ASW The growth stage and root characteristics mainly contribute to withstand S-W depletion